Biaxially stretched polyester films, especially biaxially stretched poly(ethylene terephthalate) films, have excellent transparency, dimensional stability, mechanical properties, electrical characteristics, gas barrier property, heat resistance, chemical resistance, etc. and, therefore, widely used as base materials for packaging, electric insulation, photography, drawing, electronic photography, magnetic recording, condensers, thermal transfer printing, etc. in various fields.
When polyester films are used as industrial base materials or final consumer products, however, polyester films are poor in slipperiness and apt to cause blocking with plastics and metals. Thus good mold release property, slipperiness and heat resistance are desired in polyester films. Under the circumstances, it is tried to provide polyester films with a coating layer to give such properties.
For instance, it has been proposed to provide the surface of a base film which contact the thermal head with a layer of a thermoplastic or thermosetting resin containing a lubricant or a mold release agent such as a surfactant, silicone oil, etc. as a stick prevention layer (Laying-Open Patent Publication Nos. 57-129789, 58-171992, 58-187396, 60-94390, 60-151096, etc.) or with a layer of a heat resistant resin such as silicon resins (Laying-Open Patent Publication Nos. 55-7467, 57-74195, etc.)
Although the sticking can be prevented by such a stick prevention layer, the lubricant or mold release agent bleeds out of the stick prevention layer to the surface and contaminates the thermal head causing adhesion of dust resulting in poor printing (skipping) or the lubricant or mold release agent is transferred to the back side of the film which has been wound up as a roll and impairs tight contact between the base film and the thermal transfer ink layer or to the recording paper. Also the adhesion of the base film and the stick prevention layer is insufficient and peel-off of the stick prevention layer causes contamination of the thermal head inviting poor printing (skipping). Further, in the techniques of the above described patent publications, there are defects that the layer is thick and apt to be uneven. When a thermosetting resin such as a silicon resin is used, a high temperature is required for drying, which causes thermal damage of the base film impairing the flatness of the film, which means deterioration of the product quality.
Also good slipperiness is required for the friction sheet (retainer sheet) placed inside of audio cassette tape recorders, which house magnetic recording tape. Insufficient slipperiness increases winding torque, which makes the running of the tape irregular causing wow and flattering, that is, poor sound quality in replaying. In rapid winding and rewinding, irregularity of running may stop the film in an extreme case. For remedy of this defect, it has been proposed to provide biaxially stretched polyester films with a layer containing a lubricant (Laying-Open Patent Publication No. 58-146065).
In the same way as in the case of thermal transfer printing, however, the lubricant layer is thick and, therefore, it is difficult to form a layer of a uniform thickness and that the flatness of the base film is impaired when coated films are dried. Further the layer is apt to pick up dust. Also the sticking of the lubricant, which bled out, to the back side of the wound-up film causes repellence of the printing ink when the printing is effected on the back side of the friction sheet and thus the adhesion of the printed layer to the polyester layer is poor resulting in poor printing.
In the transfer printing, which is a representative method for printing images, characters and wood grains, etc. on the surface of molded plastic products, good inter-layer adherence, easy separation of the layer to be transferred and good surface property of the transferred are required. It has been known that the substrate film is laminated with a coating or a film of a curable silicon resin having mold release property in order to give the film the mold release property.
Laminating a curable silicon resin layer, however, has problems that the coating layer is thick, the thickness is apt to be nonuniform, curing requires a high temperature, and the flatness of the film is impaired as described above resulting in poor productivity and high cost of the products. Further, there is a problem that the silicon resin has too good a mold release property and, therefore, the transferred layer easily peels off in the steps other than the transfer printing.
The present invention is to provide an inexpensive biaxially stretched polyester film having a coating layer provided with excellent mold release property, slipperiness, heat resistance, flatness and non-sticking to the back side when wound up.